Nikon SB-900 Foot: Size 16 EEEE
But before you buy, you might want to check this thread for a running update on what the flash shoes the newly designed foot will -- and won't -- fit. Please feel free to add to this list if you find out anything first hand.
And remember that they do ship with an AS-21 foot, which can be swapped out if your system is 1/4 x 20-based. But mind that plastic thread if you are swapping out frequently.
:: SB-900 Review ::